(1) Field of the Invention
The present invention relates to a high speed printing system and, more particularly, to a means for controlling a variable intensity printing applied to a record media.
(2) The Prior Art
In a printing system, it is necessary to vary the intensity of the printing which is applied to the record media in accordance with the size of the surface area of the characters. This is done in order to obtain high quality printed characters, having uniform deepness, regardless of the size of the surface area of the characters. In one prior art printing system a single control mode is employed for hammering each type element of the printer. In the single control mode, an energizing current having a constant amplitude is supplied to a hammer means during the flight of each type element toward a platen. However, the energizing current varies only when a type element selected to be hammered requires a respective predetermined printing intensity. The above mentioned prior art printing system has the following disadvantage: it is difficult to carry out a fine control of the printing impact and, accordingly, a fine control of the deepness. This is because, although the energizing current is slightly varied, the hammering speed of the type element at the platen and the flight time of the type element are widely varied.
Generally, there are two methods for hammering the type elements. In a first method, the hammering operation of a selected type element and the spacing operation of a carrier are performed alternately. This is the so-called intermittent printing method. The carrier contains a plurality of type elements and traverses back and forth along lines of the record media. On the other hand, in a second method, the hammering operation and the spacing operation are performed simultaneously. This is the so-called continuous printing method. That is, in the above mentioned first method, the carrier stops traversing every time it is located at the predetermined printing position and, then, the hammering operation follows; while, in the above mentioned second method, the hammering operation has commenced before the carrier reaches the predetermined printing position and, when the carrier reaches this printing position, the selected type on the carrier is impacted at the printing position on the record media. Therefore, the above mentioned second method is more suitable for employment in a high speed printing system than the above mentioned first method.
In a printing system employing either the first method or the second method the above-mentioned disadvantage arises when the single control mode is used to control the printing impact. As mentioned above, the disadvantage is that, although the energizing current is slightly varied, the intensity of the printing impact is widely varied, and as a result, fine control of the printing impact, and accordingly, fine control of the contrast appearing on the record media, can not be achieved. Furthermore, in a printing system employing the above mentioned second method the following disadvantage is created: the selected type element does not impact correctly at a predetermined printing position on the record media. This is because, although the energizing current is slightly varied, the flight time of the selected type element is widely varied.